The present invention relates to an organic EL display apparatus and a method of driving the display apparatus.
The present application claims priority from Japanese Applications No. 2003-394396, the disclosures of which are incorporated herein by reference.
An organic EL (Electroluminescent) display apparatus is a self-light-emission type flat panel display apparatus which attracts a high public attention because it can effectively inhibit its power consumption as compared with a liquid crystal display apparatus using a back light, and also because such an organic EL display apparatus can display a picture with a high brightness. In fact, such an organic EL display apparatus is formed by arranging a plurality of organic EL devices (luminescent devices) in an array of dot matrix, thus making it possible to display a picture by selectively effecting the luminescence of the organic EL devices.
FIG. 1 (conventional technique) shows a fundamental structure and an equivalent circuit for an organic EL device. As shown, an organic EL device 1 comprises a lower electrode 3, an upper electrode 5, and a single-layered or multi-layered organic material layer 4 interposed between the two electrodes, all being mounted on a substrate 2, thereby forming a structure in which the organic material layer 4 containing a luminescent layer is sandwiched between a pair of electrodes (3 and 5), as shown in FIG. 1A. In detail, one of the lower electrode 3 and the upper electrode 5 is used as an anode, while the other is used as a cathode. Once a forward voltage (positive charges (+) are collected on the anode side, while negative charges (−) are collected on the cathode side) is applied between the two electrodes, the electrons injected and transported from the cathode side will re-combined with the holes injected and transported from the anode side, thus effecting a light emission and allowing an electric current to flow from the anode to the cathode. At this time, electric charges are accumulated between the two electrodes until the re-combination is effected. On the other hand, if a reverse voltage (negative charges (−) are applied to the anode side, while positive charges (+) are applied to the cathode side) is applied between the two electrodes, the organic material layer 4 will function as a dielectric layer, and current will not flow between the two electrodes. Therefore, if such an organic EL device 1 is expressed by an equivalent circuit, it can be expressed by a circuit including a diode E and a capacitor C connected in parallel with each other, as shown in FIG. 1B.
When a picture is to be displayed on an organic EL display apparatus including a plurality of such organic EL devices 1 arranged in an array of dot matrix, a forward voltage is applied for a predetermined time period to some organic EL devices 1 selected corresponding to the picture to be displayed, thus allowing an electric current to flow from the anodes to the cathodes of the organic EL devices 1, thereby lighting these organic EL devices 1. Then, when lighting or non-lighting is changed over from each other with the passing of time, a delay will occur in ON-OFF of the current flowing to the organic EL devices due to a time constant of capacitor component, resulting in a phenomenon that brightness are remaining in some organic EL devices even if they should be in a non-lighted state.
In a passive driving type organic EL display apparatus, lower electrodes 3 and upper electrodes 4 are respectively formed into strips and arranged to be orthogonal to one another, thereby forming one organic EL device 1 at each intersection. In this manner, the lower electrodes are used as scanning electrodes while the upper electrodes are used as driving electrodes. Alternatively, the upper electrodes are used as scanning electrodes while the lower electrodes are used as driving electrodes. In displaying, the scanning electrodes are successively selected at a predetermined time interval so as to perform a scanning operation, while the driving electrodes are subjected to voltages corresponding to image signals in synchronism with the scanning operation, thus effecting a driving operation. In this way, the organic EL devices 1 are selectively lighted corresponding to an image to be displayed. However, in such an organic EL display apparatus, since other EL devices are present in the flowing path of an electric current generated when the selected organic EL devices are lighted, there is a problem that a forward current will flow into non-selected organic EL devices 1, hence undesirably resulting in a cross-talk light emission.
To solve the above problem, an organic EL display apparatus has usually adopted an improved driving manner such that a reverse voltage is applied to EL devices not to be lighted, while a forward voltage is applied only to those organic EL devices selected in accordance with image signals. Further, Japanese Unexamined Patent Application Publication No. 11-305727 discloses the above-mentioned passive driving type organic EL display apparatus in which a voltage (which is a reverse voltage) in an opposite direction to a voltage (which is a forward voltage) applied during light emission is applied to all the organic EL devices 1 during a predetermined time period, thereby preventing a poor light emission possibly caused due to a leak current.
According to the above-mentioned patent application publication, if a low resistant thin film portion exists in the organic material layer of the organic EL devices, a reverse voltage to be applied for preventing the above-discussed crosstalk will cause an electric current to flow only into this portion of the organic material layer. As a result, the thin film portion and its surrounding areas of the organic material layer are evaporated so that an expansion pressure occurs which forces the cathodes to be bent in a direction away from the anodes. When such an expansion progress still further, the cathodes will be damaged and this can cause an undesired insulation.
Accordingly, if a thin film portion in which anodes and cathodes are positioned extremely close to each other is present in the organic material layer, an insulation formed by applying a reverse voltage will provide a self-repair function, thereby preventing a short circuit between the anodes and the cathodes in this portion, thus inhibiting the occurrence of a leak current possibly caused by such short circuit.
However, with regard to respective organic EL devices in an organic EL display apparatus, due to various unfavorable factors existing in the manufacturing process, an un-uniformity will be locally present in the organic material layer so that it is impossible to ensure the a fore-mentioned self-repair function (namely, it is impossible to obtain an insulation by a reverse voltage).
On the other hand, since an organic EL display apparatus is required to have an increasingly higher brightness by the market, a forward voltage to be applied when lighting the organic EL devices has become considerably higher. Further, in case of a passive driving type organic EL display apparatus which is required to have a high precision when displaying an image, the light emission brightness of an EL display panel has to be greatly increased in order to deal with a decreased numerical aperture and an increased number of scanning electrode lines. Besides, in response to an increased forward voltage, it is inevitable to have an increased reverse voltage to be applied for eliminating the aforementioned problem caused by the time constant of the capacitor component and another problem caused by the crosstalk light emission.
As to the setting of an applied voltage for an organic EL display apparatus, if the above-mentioned un-uniformity exists in the organic material layer, the organic EL display apparatus can still operate normally at its initial stage of operation without any problem. However, with the passing of a certain time period, it was found that a leak current occurs due to an applied reverse voltage.
Here, it can be considered that the occurrence of the leak current caused due to an un-uniformity in the organic material layer may be avoided by improving a film formation precision of the organic material layer. However, if such an improvement is to be realized, the manufacturing technique at present time will cause a decreased product yield, resulting in an increased manufacturing cost.